1. INTRODUCTION
Optical communications technologies have continued to
advance and mature since the early 1980s. Due to their high
performance and decreasing costs, optical networks are now
being considered for use in emerging avionics systems. As
with any new technology, an efficient, accurate, and cost-
effective method for investigating and evaluating candidate
systems is needed, and computer-based simulation with rapid
virtual prototyping can be ideal. With the proper set of tools,
a combination of low-level details (e.g. link budgets) and
high-level network concepts (e.g. QoS) can be integrated to
obtain a detailed design evaluation. This paper introduces a
library for simulation modeling of optical communication
networks, focusing on performance analysis for advanced
aerospace platforms, and features a case study of virtual proto-
typing for comparison of several system design strategies.
2. LIBRARY FOR INTEGRATED OPTICAL NETWORKS
Advances in commercial optical network technology and
increased application demands have led avionics network de-
signers to explore the use of optical components in future avi-
onics systems. However, due to the high cost of prototyping
and integration with existing systems, a simulation environ-
ment to perform tradeoff analysis can be the most cost-
effective approach. A careful review of existing simulation
tools revealed that no single tool could fulfill these require-
ments without modification. Therefore, a new optical compo-
nent library was developed and added to an existing commer-
cial tool to bridge the gap between optical-centric and
network-centric simulation and performance analysis tools.
The Library for Integrated Optical Networks (LION) pro-
vides researchers with an extendable tool to assess both lower-
and upper-layer networking issues simultaneously by provid-
ing a set of accurate optical components within a powerful
network simulation environment. Developed at the University
of Florida, LION was created in a discrete-event simulation
environment called MLDesigner from MLDesign Technolo-
gies. The latest version of the library consists of 39 compo-
nents in 13 categories, such as couplers, splitters, filters, am-
plifiers, etc. These components allow for accurate modeling
of timing and some physical effects inside optical devices.
LION provides users with flexibility in designing systems
with varying high-level components and architectures. New
and legacy network protocols can be implemented on top of
components to realize and evaluate almost any system design.

This work was made possible by support from Rockwell Collins Inc. in
Cedar Rapids, IA, and MLDesign Technologies in Palo Alto, CA.

3. AIRCRAFT LAN CASE STUDY
A tradeoff study is performed to compare the cost vs. per-
formance of new and emerging technologies for high-speed
optical networking for future military avionics systems to
demonstrate LION's features. We developed models of an
aircraft LAN to compare today's technology with several vir-
tual prototype designs. The baseline system consists of typical
aircraft networking components connected with a switched
Ethernet architecture, which is considered state-of-the-art on
modem aircraft. Our two candidate models represent a future
direction for aircraft interconnects where all components are
interconnected via a switchless optical backbone using wave-
length-division multiplexing, or WDM.
An example configuration with traffic layout and set of
communication patterns between aircraft modules is used to
stimulate the network of each system. Thirty-nine Virtual
Link Identifiers (VLIDs) are defined for communication be-
tween these modules, with source traffic types including
bursty, random, periodic, and continuous, and source data
rates ranging between 10 Kb/s and 250 Mb/s. The aggregate
throughput (i.e. total offered load) required for the aircraft
LAN network traffic used in this study is 1.27 Gb/s.
MiSSiOn PCS sg Display Pressingn FR eSe Naio ver Nvig n Subsystem

|a 44 .4 |j |e

Figure 1. Candidate WDM Aircraft LAN

In the baseline aircraft LAN, the Line-Replaceable Mod-
ules (LRMs) that constitute a typical aircraft system are con-
nected with three 16-port IP/Etheret switches (Gigabit or Fast
Ethernet, as traffic requirements dictate), which support multi-
cast, buffered queues, and QoS. The system could be built
with only two switches, but would require additional cable
lengths and leave no room for scaling. Nodes in the network
all have unique IP and MAC addresses. Each VLID is associ-
ated with a set of QoS requirements and a unique multicast
address. Routing is performed based on QoS demands.
In the candidate WDM aircraft LAN, as shown in Figure 1,
the same mixture of LRMs used in the baseline system is also
used to stimulate this system. Two WDM systems are derived

1 of2

from this configuration and both feature a switchless optical
backbone to interconnect modules, with traffic assigned to one
of seven wavelengths, and all transmitters operating at 1 Gb/s.
In WDM System A, each node has one fixed-wavelength laser
and five optical filters in the receiver. Wavelengths are as-
signed by subsystem (e.g. all sensor subsystem nodes are as-
signed wavelength 1, all communication subsystem nodes are
assigned wavelength 2, etc.). In WDM System B, each node
is equipped with two fixed-wavelength lasers, but only three
optical filters are needed in the receiver. Here, wavelengths
are assigned to VLID groups, with most inter-subsystem traf-
fic contained within a single wavelength. While System A
only requires a single laser per node, an additional filter is
needed for traffic received from a different subsystem.
Table 1: Summary of Case-Study Results
Baseline WDM WDM
Attribute
Att e System System A System B
Average End-to-End
Traffic Latency (gs) 5,128.96 119.46 87.50
High-Priority Traffic (gs) 141.35 112.49 80.93
Medium-Priority Traffic (gs) 5,613.18 127.22 84.82
Low-Priority Traffic (gs) 8,137.90 505.61 491.51
Standard Deviation of
Overall Latency (gs) 3,352.69 70.87 48.67
Standard Deviation of High-
Priority Traffic Latency (gs) 99.11 66.03 43.48
Theoretical Aggregate System
Throughput (Gb/s) 10.20 7.00 7.00

The results of injecting the traffic patterns onto the baseline
and the two WDM architectures are summarized in Table 1.
Both WDM systems produce average latencies that are sig-
nificantly better than those observed in the baseline system.
The performance of the baseline system suffers due to in-
creased overhead requirements, and traffic contention at heav-
ily congested destination nodes resulting from traffic load im-
balance. Unlike the Ethernet case, several traffic streams
simultaneously headed for the same destination cause no con-
tention in the WDM systems. Messages in the WDM systems
do not require packet processing while in transit, as opposed to
packets in the baseline system which are processed and de-
layed at each switch for routing and QoS purposes.

In addition to lower averages, the jitter experienced by the
traffic (i.e. variance in latency) is also observed to be better in
the WDM systems. Using time-division multiplexing (TDM)
as the access method within each wavelength, expected wait-
ing periods are more deterministic compared to the switched-
Ethernet system. This behavior is vital in military applications
with mission-critical requirements. Moreover, much of the
jitter experienced by high-priority traffic on the WDM sys-
tems was observed to be caused by delivery under the mean
time. The WDM systems are also potentially more scalable
than the switched-Ethernet network. Instead of requiring more
switches and ports, additional wavelengths can be employed in
the optical backbone. The use of faster components is another
option, since many WDM systems today operate at 10 Gb/s
per wavelength instead of the 1 Gb/s rates used in this study.

A significant difference was also seen between the two
WDM systems under study. System B achieved an overall
average latency approximately 30 gs below that of System A.

As shown in Figure 2, System B delivers more packets in less
than 100 ps, and far fewer in the 100-300 gs range, which is a
major reason why lower variances are seen in system B as
well. Although difficult to ascertain from this chart, System A
produced no latencies over 1 ms, as opposed to -0.1% for Sys-
tem B. The difference in performance can be largely attrib-
uted to the flexibility that the extra laser per node in System B
offers in balancing the traffic efficiently between wavelengths.
The low-priority traffic streams suffer little, since they occupy
fewer TDM slots. In System A, the higher-priority traffic was
often assigned a smaller portion of time slots than in System B
due to wavelength load imbalance. Designers will have to
decide if the gain in performance seen in the approach of
WDM System B is worth the extra hardware, especially when
fault-tolerance needs might dictate the requirement for a du-
plicate of every communications component (e.g. laser) in the
system. This case study merely represents a small subset of
one example of how simulation with LION can be used to
design and evaluate future optical networks for aerospace.

A brief description of the LION library including its moti-
vation and components was provided. Three examples of an
aircraft LAN network were presented to demonstrate system-
level modeling and performance tradeoffs using the tool. The
examples included a switched-Ethernet system and two WDM
optical backbone systems. Results from the case study dem-
onstrated that WDM networks offer superior performance,
scalability, and global synchronization. The simulation mod-
eling approach we describe is not limited only to military avi-
onics networks it is applicable to other design alternatives
for complex communication systems where optical network-
ing can offer advantages, such as commercial aircraft, ships,
automobiles, communications and computing centers, etc.
Future directions for research include expanding the LION
component library and undertaking a broader range of system
case studies, including design factors such as fault tolerance,
as well as alternate control schemes and protocols.

5. ACKNOWLEDGEMENTS

Several former students in our lab made substantial contri-
butions to models in LION, including Jeremy Wills, Chalerm-
pong Dilakanont, Chris Catoe, and Ramesh Balasubramanian.